The present invention relates to a process for the transesterification of keto esters. More particularly, the present invention relates to a process for the trans esterification of ketoesters using solid acids as catalysts. The solid acids used in the present invention may be either sulfated zirconia, sulfated tin oxide, sulfated titania, sulfated iron oxide, any heteropoly acid, acidic clay, acidic zeolite, like H-ZSM 5, HY, etc. or any other solid acid with high acidity or super acidity. The process for the preparation of the solid acids has been fully described in our co-pending U.S. patent application Ser. No. 08/653,171 filed May 24, 1996, which includes the following disclosure.
The present invention relates to an improved process for the preparation of sulphated mixed metal oxides. More particularly, the present invention relates to a process for the preparation of sulfated mixed metal oxides having the general formula MxN1xe2x88x92xO2 where M is one of the group IIIB metals such as yttrium, scandium or lanthanum and N may be either Zr, Ti or Sn and xe2x80x98xxe2x80x99 may vary from 0.01 to 0.4. The sulphated mixed metal oxides termed as solid acids or super acids or modified super acids, prepared according to the process of the present invention is useful as a catalyst for various organic transformations such as, trans esterification, Diels-Alder reactions, Eno reactions etc.
Solid acids are well known in the field of catalysis since the pioneering work of G. A. Olah, who developed strong liquid acids such as sulphuric acid, aluminum chloride etc., (G. A. Olah, U.S. Pat. No. 4,116,880, 1970) later on replaced by solid superacids such as sulphated zirconia, sulphated titania etc. Several modifications of such solid acid materials have been attempted in the prior art recently. Hsu et al., reported that Fe and Mn promoted sulphated zirconia catalyst shows higher stability than pure sulphated zirconia and that this new catalyst can isomerize n-butane at near room temperature with a rate three orders of magnitude greater than sulphated zirconia (E. J. Hollstein, J. T. Wei and C. Y. Hsu, U.S. Pat. No. 4,918,041 (1990). Apart from this there are several other isomerization reactions promoted by solid super acids. For example, Kramer [U.S. Pat. No. 4,357,484 (1984)] disclosed an isomerization process where adamantine is added to a halide containing Lewis acid catalyst and it appears that carbonium intermediate must be generated in solution to effect the isomerization reaction. A process for the isomerization of cyclic hydrocarbons using liquid acids such as sulfuric acid or fluoro sufonic acid in the presence of adamantine has been described in U.S. Pat. No. 3,871,598 (1972).
There are several other areas in the field of chemistry where these superacids or modified superacids finds applications. A mention must be made here of industry application of solid acids as catalysts in catalysing alkylation, dewaxing, fluidized catalytic cracking, hydrocracking, hydrotreating, isomerization and reforming, Beckman rearrangement, Fries rearrangement etc.
Attempts have also been made in the prior art to use solid acids for catalysing Diels-Alder reactions/cyclo-additions, and other allied reactions using several Lewis acids such as T1C14, SnC12, ZnC1z (complexed with ethers), ZnBr2, BF3, Eu(III) complexes, YB(III), Sc(III) and XI(III) complexes. However, there are several limitations in using these as catalysts because of poor endo product selectivity, difficulty in recovery of the catalyst, handling problems and lack of reusability.
In view of the above mentioned limitations of the prior art process it was desirable to develop some solid acid catalysts which have properties similar to that of the known liquid super acids or lewis acid anchored with metal complexes, which not only possess high acidity but also have stable structure and useful in various organic transformations.
The objective of the present invention is therefore to develop a solid acid capable of being used as a catalyst in Cxe2x80x94C bond forming reaction, which is active either at room temperature or at a higher temperature, easy to recover, reusable, and selectively in synthesis of endo products in Diles-Alders reactions.
The present invention relates to preparation of a family of stable solid acids which may also be designated as solid solutions the surface of which are modified with sulfates, having very high acidity and they can be identified as xe2x80x9cSuperacidic Solid Solutionxe2x80x9d(SSS). The xe2x80x9cSSSxe2x80x9d possesses strong Lewis acidity and also Bronsted acidity to some extent as demonstrated by infra red spectra of adsorbed pyridine and Temperature Programmed Desorption of Ammonia and potentiometric titration of n-butylamine in nonaqueous media.
Accordingly, the present invention provides a process for the preparation of sulphated metal oxides which comprises:
a) preparing an aqueous solution A of one of the metal salts of zirconium, tin, titanium or iron the strength of which ranging from 0.1 to 2 M,
b) preparing an aqueous solution B of group III B metal salts (such as metal salts or yttrium, scandium or lanthanum) the strength of which ranging from 0.1 to 2 M,
c) mixing solutions A and B in the molar ratio of P:Q where, p ranging from 99 to 60 and q ranging from 1 to 40 and p+q always being equal to 100, to form solution C,
d) precipitating solution C to form hydroxide D by adding ammonium hydroxide or tetraalkyl ammoniumhydroxide where, the alkyl group is selected from methyl, ethyl, propyl or butyl, drying the precipitate D formed at a temperature ranging from 90 to 110xc2x0 C. to form a dry powder designated as E,
e) treating the powder E with sulfuric acid or ammonium sulfate of strength ranging from 0.1 to 4 M for a period ranging from 1 to 12 hours followed by heating the resulting product at a temperature ranging between
40 to 110xc2x0 C. at atmospheric pressure, or also at reduced pressure to get a dry powder designated as F,
f) calcining the dry powder F at a temperature ranging from 300 to 500xc2x0 C. for 3 to 12 hours to get sulphated metal oxides catalyst.
In the preferred embodiment of the present invention, such superacidic solid solution mainly consists of one among the zirconia, titania, tin oxide or iron oxide in major amount, (hereinafter called xe2x80x98support metal oxidexe2x80x99) and may contain at least one or all of the group IIIb rare earth metals like scandium, yttrium or Lantahnum (hereinafter called xe2x80x9cdopant metaloxidexe2x80x9d) and the support metal oxide forms solid solution with dopant metaloxide(s) when the latter is co-precipitated or co-gelated with the former followed by calcination.
In another embodiment of the present invention, the support metal oxide is treated with stabilizing anions preferably sulfates before calcination, while the source of sulfates are from SO2, SO3, H2S, H2SO4, or ammonium sulfate and the amount of sulfates may range between 0.1 to 4% (weight % as sulfates) of the total weight of the catalyst.
In yet another embodiment of the present invention, the superacidic solid solution have general formula, MxN1xe2x88x92xO2 where x may range from 0.1 to 0.40 which means amount of dopant metal oxide content may vary from 1 to 40 mole % and amount of support metaloxide content may vary from 99 to 60 mole %. In other words, in the extreme cases, it may either have merely support oxide doped with sulphates in which case it can be conveniently called as xe2x80x9csuperacidsxe2x80x9d instead of the name xe2x80x9csuperacidic solid solutionxe2x80x9d or it may also have separate phases of support and dopant oxides in which case it may be called as xe2x80x9csuperacidic mixed oxides.xe2x80x9d
According to one feature of the present invention, the source of the group III B metal oxide may be from their respective salts such as nitrates, chlorides, acetates, or sulphates or their alkoxides. For example it may be either Yttrium nitrate, Scandium nitrate, Lanthanum acetate, Lanthanum chlorides, etc.
According to another feature of the present invention, the source of sulphates may also be gases such as SO3 or SO2 in which case the powder D is exposed to these gases for time ranging from 3 to 24 hours.
The catalysts prepared according to the process of the present invention is active in catalysing reactions such asxe2x80x94C coupling reaction (Diels-Alders reaction or Hetero-Diels-Alders reaction or inverse electron demand Diels-Alders reaction or one reaction), transesterification and protection of carbonyl groups and deprotection of allyl esters. Some of these processes are described in our co-pending patent application No. NF 175/95 and NF 177/95.